The present invention is in the field of electrical heating and relates to electrical heating units of the so-called integral element type comprising a glass or other ceramic plate or block as the heating surface, which plate or block is heated by an electrical heating element bonded to and supported thereby. Such heating units are particularly useful for electrical cooking ranges, hot plates, and other electrical heating applicances.
U.S. Pat. No. 3,086,101 discloses an electrical heating unit of the discrete element type, comprising a glass plate having an electrical heating element positioned in physical contact with the lower surface of the plate. The unit may optionally include an alumina coating between the heating element and the plate to prevent chemical interaction therebetween during use at elevated temperatures.
U.S. Pat. No. 3,067,315 discloses an electrical heating unit of improved heating characteristics comprising a high silica glass plate having directly bonded thereto a thin noble metal film which acts as the electrical heating element of the unit. However, supporting plates having lower optical transparency and increased impact strength are desired.
Since the discovery of the so-called glass-ceramic family of ceramic materials, such as described in U.S. Pat. No. 2,920,971, electrical heating units comprising glass-ceramic heating plates have been developed. The strength, low porosity, and excellent thermal properties of some of these glass-ceramic materials have provided electric ranges and other electrical heating units of excellent appearance and cleanability. Up to the present time, however, such units have generally been of the discrete electrical element type, such as described in British Pat. No. 1,391,076 and U.S. Pat. No. 3,889,021, wherein the electrical heating element is not directly bonded to but is simply in close proximity to the glass-ceramic plate to be heated. Numerous problems are associated with the development of heating units comprising electric heating elements integrally bonded to the glass-ceramic heating plate.
One of the most important requirements of a glass-ceramic material to be utilized as a burner plate for an electrical heating unit is high strength. Such plates may be subjected to heavy impacts in use, and the cost of replacement of the entire plate upon breakage is prohibitive. Glass-ceramic materials normally exhibit higher modulus of rupture strengths than glasses; hence glass-ceramic electrical heating units of the discrete element type typically exhibit adequate resistance to breakage on impact.
Examples of glass-ceramic materials exhibiting properties rendering them particularly suitable for this use are the beta quartz-zinc petalite glass-ceramic materials described in U.S. Pat. No. 3,681,097 to Beall and Martin. Glass-ceramics of this type are known which exhibit modulus of rupture strengths in excess of about 15,000 psi, average linear coefficients of thermal expansion in the range of about -5 to +20 .times. 10.sup..sup.-7 /.degree.C. over the range from 0.degree.-800.degree.C., good opacity, and excellent chemical durability. Moreover, these glass-ceramics exhibit high electrical resistivity, even at elevated temperatures, such that additional electrical barrier layers to minimize electrical leakage through the material from a bonded heating element would not be required. Hence, it was expected that electrical heating elements could be directly bonded to the surface of these glass-ceramics to provide efficient heating units.
However, we have discovered that the modulus of rupture strength of beta quartz-zinc petalite glass ceramics can be substantially reduced when it is attempted to bond electrical heating elements consisting of electrically conductive films directly to the glass-ceramic surface. This problem is particularly severe with cermet films comprising conductive metallic constituents in combination with ceramic binders, but is also observed to some extent with glass-free metallic films.
A related problem which has been encountered pertains to the difficulty of obtaining good bonding between glass-free metallic films and the smooth glass-ceramic surface. Attempts to solve this problem have included the application of ceramic frits to the glass-ceramic surface prior to metal film application, in order to provide a somewhat roughened surface finish. However, these frits also have exhibited a tendency to interact with the glass-ceramic during the application process, and to thereby weaken the plate.
Thus, the major problems of bonding electrically-conductive heating elements to the surfaces of beta quartz-zinc petalite glass-ceramics center around an incompatibility between these glass-ceramics and the metallic, ceramic, and cermet materials which must be bonded thereto in order to provide an integral heating element.